The present invention relates to a method for monitoring the development of living biological samples and, more particularly, to an apparatus and method for monitoring the development of incubating cell cultures such as embryos.
Cell cultures commonly are grown inside incubators. An incubator is a closed box within which t he e nvironmental parameters, such as temperature, humidity and atmospheric composition, can be optimized to promote the growth of the cell cultures. For example, mammalian embryos should be incubated under conditions resembling those found inside a mammalian womb.
It is advantageous to monitor the growth of cell cultures using a microscope, so that the details of the development of individual cells may be observed. If a photomicroscope is used, it can be focused on successive focal image planes within the culture, in a manner similar to that taught by Carlsson in U.S. Pat. No. 4,631,581 for microphotometry of prepared biological specimens, to record successive two-dimensional slices through the cell culture, thereby obtaining a three-dimensional record of the structure of the cell culture. Because microscopes are large, bulky, delicate instruments that do not fit inside commonly used incubators, it has been the practice heretofore to enclose microscope stages in specially constructed incubators so that those microscopes could be used to monitor incubating cells. This clearly is an awkward procedure. Furthermore, this procedure allows only one cell culture, or only one portion of a cell culture, to be monitored within the incubator at any given time.
This problem is addressed partially by Miyamoto in U.S. Pat. No. 5,307,161. Miyamoto places a solid-state area image sensor array, such as a charge coupled device (CCD) array, in close proximity to a biological sample within an incubator. CCD arrays are small enough to fit inside commonly used incubators. If positioned close enough to the biological sample, a CCD array does not need an optical system in order to image the sample. Several biological samples may be monitored simultaneously, each by its own CCD array. Signals from the CCD array are transmitted to a display unit such as a video monitor, and also may be digitized, processed and stored in the conventional manner. The drawback of using a solid-state area image sensor array without an optical system, as taught by Miyamoto, is that the resolution of the images obtained is limited by the size of the sensor elements that comprise the array. Sample features smaller than the width of one sensor element cannot be imaged.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method for microscopic monitoring of a living cell culture within a conventional incubator.